1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit and a method for driving the same, to drive a plurality of fluorescent lamps by one inverter, and to prevent wave noise.
2. Discussion of the Related Art
A cathode ray tube (CRT) has been widely used as a monitor of a television, a measuring apparatus, and an information terminal, such as for a personal computer. However, the CRT is not compact in size or light in weight. Thus, various alternative display devices have been developed. For example, a liquid crystal display (LCD) device using an electric field optical effect, a plasma display panel (PDP) using gas discharge, and an electroluminescence display (ELD) device using an electric field luminous effect, have been employed as substitutes for the CRT.
Among the various substitute display devices, the LCD device has been most extensively researched. The LCD device has low power consumption, is slim, and is lightweight. The LCD device is in active development and is being used as a monitor for desktop computers (or personal computers) and large sized display devices, as well as laptop computers (or notebook computers). Accordingly, LCD devices are continuously in demand. Most LCD devices control light transmittance of ambient light to display an image. In this respect, it is necessary to form an additional light source, such as a backlight unit, for an LCD panel.
Generally, the backlight unit used as the light source of the LCD device is classified into two types, namely a direct type or an edge type, according to the arrangement of the fluorescent lamps thereof.
In the edge type backlight unit, a lamp unit is provided at a lateral side of a light-guiding plate. The lamp unit is provided with a fluorescent lamp for emitting light. A lamp holder holds both ends of the fluorescent lamp for protection of the fluorescent lamp. A reflective sheet reflects the light emitted from the fluorescent lamp to the light-guiding plate.
The edge type backlight unit is generally used in relatively small-sized LCD devices, such as monitors of laptop computers and desktop computers, since the edge type backlight is advantageous in that it provides light uniformity, a long lifespan, and allows for a thin profile of the LCD device.
The present trend is to produce large-sized LCD devices, e.g. of 20-inch or more. For large-sized LCD devices, the direct type backlight unit is actively developed, in which a plurality of lamps are formed in one line on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with the light produced by the lamps. A direct type backlight unit is used for a large-sized LCD device because the large-sized LCD requires a high luminance. The direct type backlight unit has greater light efficiency, as compared with the light efficiency of the edge-type backlight unit.
Hereinafter, a backlight unit, in accordance with the background art, will be described with reference to the accompanying drawings. FIG. 1 is a plan view illustrating an arrangement of a backlight unit according to one method of the background art. FIG. 2 is a plan view of illustrating an arrangement of a backlight unit according to another method of the background art.
As shown in FIG. 1, the backlight unit, according to one method of the background art, includes a lamp housing 10. The lamp housing 10 is provided with a plurality of fluorescent lamps 12 arranged at fixed intervals, wherein each fluorescent lamp 12 has first and second electrodes 11a and 11b formed at respective ends of a tube thereof. First and second power supplying lines 13a and 13b are provided at the first and second electrodes 11a and 11b to supply power thereto. A condenser 14 is connected with each the first power supplying lines 13a for each of the fluorescent lamps 12. A first end of an output coil of a transformer 15 is connected with the condenser 14, and a second end of the output coil is connected to the second power supplying line 13b. The second power supplying lines 13b are grounded with the lamp housing 10.
As shown in FIG. 2, the backlight unit, according to another method of the background art, includes a lamp housing 20. The lamp housing 20 is provided with a plurality of fluorescent lamps 22 arranged at fixed intervals, wherein each fluorescent lamp 22 has first and second electrodes 21a and 21b formed at ends of a tube thereof. First and second power supplying lines 23a and 23b are provided at the first and second electrodes 21a and 21b to supply power thereto. First and second condensers 24a and 24b are respectively connected with the first and second power supplying lines 23a and 23b. The first condensers 24a are connected with one end of an output coil of a first transformer 25a in common. The second condensers 24b are connected with one end of an output coil of a second transformer 25b in common. The other ends of the output coils of the first and second transformers 25a and 25b are grounded with the lamp housing 20.
In the aforementioned backlight unit according to another method of the background art as shown in FIG. 2, positive and negative polarity voltages having the same level are respectively applied to the first and second electrodes 21a and 21b. As a result, the same phase voltage is applied to the first or second electrodes 21a or 21b. For example, if the positive polarity voltage is applied to all the first electrodes 21a, the negative polarity voltage is applied to all the second electrodes 21b. Conversely, if the negative polarity voltage is applied to all the first electrodes 21a, the positive polarity voltage is applied to all the second electrodes 21b. 
In FIG. 1 and FIG. 2, the condensers 14, 24a and 24b prevent any sharp increase of discharge current when separately driving the fluorescent lamps. Also, in case of driving the plurality of fluorescent lamps connected in parallel by one power device, the condensers uniformly divide the current, thereby obtaining uniform luminance in the respective fluorescent lamps.
However, the background art backlight unit has several drawbacks. In the case of the backlight unit of FIG. 1, the condenser is connected with one electrode of each of the fluorescent lamps. In this state, a high voltage is applied to one electrode in each fluorescent lamp, and the other electrode of the fluorescent lamp is grounded, whereby the backlight unit is driven by a high-low method. That is, the electrode having the high voltage is firstly luminous, and then it is luminous toward the grounded electrode. Accordingly, the electrode portion of the fluorescent lamp to which the high voltage is applied is brighter than the grounded electrode portion of the fluorescent lamp, so that it is impossible to realize a uniform luminance in the fluorescent lamp.
In the case of the backlight unit of FIG. 2, the condensers 24a and 24b are respectively connected with both ends of each of the fluorescent lamps, whereby the condensers and the electrodes are provided in parallel. In this arrangement, if the same phase voltage is applied to the electrodes provided at the same side of the backlight unit, it may generate bit frequency due to frequency interference between the fluorescent lamps arranged at the same side of the backlight unit, thereby causing noise. Accordingly, when the backlight unit is mounted to an LCD panel, wave noise may be generated due to the noise of the backlight unit.